As the topic states, does anyone have any insight into strapping a heptode as a triode? Since G2 and G4 are tied together internally, and G5 is tied to cathode, acting as a supressor grid, I suppose strapping G2+4 and G3 to anode would do the trick? I have seen this done in a VTVM (possibly in order to save money by using commonly available tubes at the time) but cannot find the schematic to make sure... please don't ask why I need this

Merry Christmas. In the case of small signal pentodes (e.g. 6AU6), the manufacturer normally recommends strapping both the screen and suppressor to the plate for triode operation. This suggests that, with hexodes, heptodes and octodes, all grids except the control grid should probably be be strapped to the plate.

An interesting exception - the data sheet for the Svetlana EL509 shows a set of curves for the tube in triode mode. but with the control and screen grids strapped together, not in the usual configuration with te screen tied to the plate. This may be done to allow a higher plate voltage.

Hehe wouldn't you like to know
Actually, I am looking into a way to use a bunch of brand new ECH21 which I have. They just look so good and I even have sockets for them. It seems a waste not to be able to use them. Surprisinhly there seem to have been no European sual triodes in loktal... (No ECC 2x, not even an ECF 2x). The cathodes of the C and H part are common which severely limits what one can do with it, and the triode does not seem very linear... so maybe the triode strapped heptode (or pentode strapped heptode, if that is possible?) may be better

Quote:

Originally posted by wrenchone
An interesting exception - the data sheet for the Svetlana EL509 shows a set of curves for the tube in triode mode. but with the control and screen grids strapped together, not in the usual configuration with te screen tied to the plate. This may be done to allow a higher plate voltage.

Yes, but you get a signifficantly different triode - higher mu but also higher rp - you really need that high plate voltage if you want to use it like that. There are more exceptions like this - for instance, the Russian 'universal pentode' 12Z1L, which was used in some of their military radios as everything except the RF power output tube, has a very interesting data sheet - it shows standard pentode and triode strapped (G2, G3 to plate) curves, but also gives you triode gm curves for G1, G2 and even G3 used as triode 'grid'. It is actually an interesting tube in many ways, being a glass envelope loktal tube inside an aluminium container which also acts as a shield.

What I found interesting was that the Svetlana EL509 data sheet showed curves for the triode mode I just mentioned with screen and control grids connected and for "enhanced triode" mode, but not for strapping screen to plate. The curves in pentode mode show a nice, big kink, so it might be better to use these guys as triodes. I may find out, as I'm angling to get four of these bad boys...

I understand what you are saying, and in fact in researching the ways a heptode can be used, I found several example where it is connected as a triode and one where it is connected as a pentode.
All in all, since g5 is tied to the cathode internally, and g2 and g4 are also tied together internally, we have the following combinations:
As pentode G2 + G4 used as screen grid and:
1. G1 tied to cathode, G3 used as the control grid (*)
2. G3 tied to cathode, G1 used as the control grid
3. G1 + G3 tied together used as control grid
4. G3 tied to plate, G1 used as control grid
5. G3 tied to G2+G4, G1 used as control grid

As triode:
1. G2+G4 and G3 tied to plate, G1 is the control grid
2. G2+G4 tied to plate, G1 tied to cathode, G3 is the control grid (*)
3. G2+G4 tied to plate, G3 tied to G1 and used as control grid (*)
4. G2+G4 and G3 tied together used as control grid, G1 tied to cathode
5. G2+G4 and G1 tied together used as control grid, G3 tied to cathode
6. G2+G4, G1 and G3 tied together used as control grid
7. G1 and G3 tied to cathode, G2+G4 used as control grid

That makes 5 different pentodes and 7 different triodes but not all are equally promising. The combinations marked with (*) I have seen in various schematic diagrams.
In particular, G2+G4 do seem to be the equivalent of a pentode screen grid, so probably pentodes 1-3 and triodes 1-3 are the useful ones.